Most mass algal cultures are operated at high enough cell density that the response of each cell is influenced by shading; one way they respond is by making additional pigment in order to be competitive in the low average light environment of the culture. Typically, the chlorophyll a content in mass algal cultures is 2-3% of the cellular mass. In such mass cultures, the cells are exposed to high light in the regions of the culture closest to the light source. Here the light is absorbed faster than it can be processed by the cell, leading to photosynthetic inefficiency.
Many efforts have been made to increase the photosynthetic efficiency, and hence the biomass productivity, of mass algal cultures through manipulation of the algal light harvesting apparatus to bring light absorption in high light into balance with the rate at which the algal cell's metabolic apparatus can process the products of the light-induced charge separation and carbon dioxide into cell mass. Normally, the imbalance under high light causes the dissipation of absorbed photon energy into heat, and as much as 90% of the photons can be wasted in this manner. This is often called light saturation.
One common way to overcome light saturation and increase photosynthetic biomass productivity is to reduce the cell's capacity to absorb light by reducing the cellular content of pigment, particularly of chlorophyll. This is typically done by inducing genetic changes and selecting cells that have tested characteristics that tend to correlate with higher productivity. Unfortunately, many times, the selected variants do not exhibit high enough efficiency under high light.
Currently various screening protocols are used to select from thousands (or more) of genetically altered variants of an algal strain to discern the few that possess criteria that one would expect to lead to greater biomass productivity under high incident light by overcoming saturation effects that waste photons. These screens include: visual or optical discrimination of low chlorophyll variants; selection of these strains; growth in low light to assure that the low chlorophyll change endures; assessment of some measure of the light intensity of saturation; and some measure of the maximum photosynthetic rate, all to assure similarity to the parent strain. These screens do not assure that the parent strain is efficient itself.
What is needed is an improved screening procedure that takes into consideration not only photosynthetic efficiency but also increased biomass/lipid/starch productivity and/or production efficiency.